For mobile wireless communication devices, many technologies have been developed to obtain and maintain a reliable link with a base station serving the wireless device.
Often, such technologies involve Adaptive Modulation and Coding (AMC). That is, the modulation method and/or data coding method used by mobile wireless communication devices (hereinafter “mobiles”) to transmit (uplink) to a base station may be based on indications from the base station of the properties (fading, interference, etc.) of the wireless channel being used for the uplink.
It is known that some of the interference experienced on the channel between the mobile and the base station may be caused by other mobiles communicating with the same base station. As such, scheduling schemes have been developed such that the mobiles in a given sector (i.e., a geographically defined area containing mobiles served by a given base station) transmit data in staggered time slots, as dictated by the base station. Such staggering of transmission times (known as time division multiplexing) allows the mobiles to use more power (and, therefore, a coding level and a modulation level that provide for a higher data rate) when transmitting in a designated time slot without concern for interference with the transmissions of other mobiles in the sector and thereby allows all of the mobiles in the sector to maintain a more robust uplink to the base station. Some systems may allow simultaneous multiple uplink transmissions from a number of mobiles and some of these mobiles may be allowed to use higher power levels, which allow for higher data rates.
In all such cases, higher power uplink transmissions have been seen to excessively interfere with the transmissions of mobiles in adjacent sectors to corresponding base stations depending on the mobile location. Such interference is seen to decrease the overall throughput of the mobile communications system. Although it may seem clear that all that is required to avoid such inter-sector interference is to coordinate scheduling between sectors, such coordination requires a complexity of design that is not favored for future mobile communications system architectures.
In another means of controlling this interference, a given base station provides feedback to each mobile on the total interference measured by the given base station over the noise power (sometimes called “Rise Over Thermal”, or “ROT”). Given an ROT target, the interference may be controlled, by instructing mobiles to decrease transmission power level, when the ROT target is exceeded. However, in general, the ROT target is fixed for all the time and it is equal for all the base stations. Unfortunately, some mobiles cannot increase transmission power level beyond a certain level and, as a consequence, these mobiles cannot increase power levels high enough to transmit using certain high data rates.
A large variation in inter-sector interference resulting from variations in time slot use by mobiles in different locations, makes estimation of carrier-to-interference ratio (C/I) in an uplink channel for a given mobile unreliable. In such a case, AMC is usually designed to provide for a large interference margin to avoid channel errors. However, a large interference margin generally lowers the capacity of both the uplink and the downlink.
Since the inter-sector interference cannot be predicted without complex inter-sector coordination, it may be considered difficult to control the fairness of the uplink transmissions. If an equal number of time slots are allocated to each mobile, the throughput of the higher C/I mobiles will be higher than those of lower C/I mobiles. This higher throughput is based on a higher data rate being available to the higher C/I mobiles for transmissions. For example, the known proportionate fair (PF) scheme requires that both “available rate” and the throughput received during the past n number of time slots be used to prioritize mobiles. However, in the PF scheme, those mobiles with a higher available rate are provided with a higher throughput than those mobiles with lower available rates. In addition, large errors in the prioritization mechanism resulting from the errors in C/I estimation used in such a scheme could lead to a fairness different than expected from the PF scheme.
Clearly, improvements are required in power management and scheduling for the uplink portion of mobile communication systems such that both intra-sector and inter-sector interference is controlled, while maintaining fairness.